1. Field of the Invention
This invention relates to a crosstalk attenuator system in which a high-frequency main signal and a low-frequency sub-signal are frequency multiplexed and recorded as a compound signal on a recording medium, and the crosstalk between the high-frequency signal and low-frequency signal is reduced at the time of separating, demodulating and reproducing the original information signals from the recording medium.
2. Description of the Prior Art
In general, there is empolyed a system for video devices (as a typical example, video disk players), in which a signal is recorded on a recording medium such as a disc on a high-density basis, that is, a video signal and a voice signal are multiplexed and recorded as a compound signal in an identical track on the recording medium and at the time of reproducing the compound signal, the original video and voice signals are separated from the compound signal.
One of the problems in such a system in which a plurality of signals are multiplexed, recorded and reproduced is that crosstalk tends to occur due to the interference between the signals.
Now, explanation will be made with reference to a typical video disk player, about how such crosstalk occurs.
On the surface of a rotary disc used in the video disk player, a spiral recording track is provided in a similar way to an audio recording disc, and a video signal and a voice signal are recorded in the track as a multiplexed compound signal. In this case, there are two ways of recording signals on the disc; one is that a spiral track is provided on the disc surface to record signals in a similar way to an audio recording disc, and the other is that there is no track on the disk surface. In an example of the recorded compound signal, the video signal is FM modulated with a carrier of about 8 MHz and the voice signal is FM modulated with a carrier of about 2 MHz.
In the above-mentioned recording process, how crosstalk occurs will be explained with reference to FIG. 1.
In FIG. 1, there is shown a video player employing a light system in which a compound signal is optically recorded on a recording medium such as a rotary disc, and later the compound signal is extracted from the rotary disc and demodulated into the original video and voice signals. In the figure, the upper part shows the recording process of the video disk player and the bottom part shows the reproducing process thereof.
A video signal is supplied to an FM modulator 1 which FM modulates it with a carrier of about 8 MHz. The output signal from the FM modulator 1 has a frequency range of about 8 MHz.+-.4 MHz. Similarly, a voice signal is applied to an FM modulator 2 where the signal is FM modulated with a carrier of about 2 MHz. The output signal from the FM modulator 2 has a frequency range of about 2 MHz.+-.0.1 MHz. The output signals from the FM modulators 1 and 2 are added together at an adder 3 and applied to a limiter 4 which shapes the added signal into a rectangular pulse.
The frequency change in the rectangular signal obtained from the limiter 4 indicates the FM composition resulting from the video signal, and the duty factor change in the rectangular signal indicates the FM composition resulting from the voice signal.
The rectangular signal is recorded into a spiral track on a rotary disc by means of a light beam recorder 5. In the recording process, the positive parts of the rectangular signal correspond to the transparent mode in which light can pass, and the negative parts thereof correspond to the opaque mode in which light can not pass.
On the other hand, in the reproducing section of the video player, the compound signal is detected from the rotary disc by the use of a light beam reader 6 and is applied to a high-pass filter 7 which extracts the repeated frequency component out of the rectangular signal. The extracted component signal is fed to an FM demodulator 9 to demodulate it into the original video signal. On the other hand, the compound signal read from the rotary disc is also applied to a band pass filter 8 which in turn takes the duty factor change out of it. The duty factor change is demodulated at an FM demodulator 10 into the original voice signal. In such a video disc player system, crosstalk is caused to occur in the above-mentioned limiter and light beam recorder 5.
First, explanation of crosstalk from the voice signal to the video signal will be given.
Assume that cos .omega.t as a frequency-modulated (FM) video signal and A cos at+X as a frequency-modulated (FM) voice signal are applied to the limiter 4. Where, A is a constant and usually is selected to be 0.1, and X is an offset of the slice level.
If the video signal is 0, then the duty factor in the rectangular signal obtained from the limiter 4 will be 50% so that the duty factors for the transparent mode and the opaque mode to be recorded on the rotary disc are each 50%.
However, the use of a photo-resist technique in the actual light beam recording process will inevitably cause deviation in the duty factor to a certain extent. The deviation usually ranges from 40% to 60% and corresponds to .+-.0.31 of offset X.
Under such conditions, if the output signal from the limiter 4 is checked in waveform, then the video FM signal will be 1 in amplitude. However, it has been found that the amplitude of the video FM signal is actually amplitude modulated with the voice FM signal and can be expressed as follows: ##EQU1##
As will be seen from the above expression, when offset X is 0, the system will generate a side band of two times the frequency (2a) of the voice FM signal frequency.
If offset X is not equal to zero, the system will generate a side band resulting from the fundamental wave of the voice FM signal, in addition to the above-mentioned side band. The modulation factor is A.sup.2 /4 with respect to the second order wave, and X.multidot.A with respect to the fundamental wave.
Now, consider that A=0.1 and X=0.31; then it will be found that the modulation factor of the side band resulting from the fundamental wave is greater by 12 times than that resulting from the second harmonic wave and is the dominant factor. The modulation factor due to the second order wave is a constant, whereas that due to the fundamental wave varies depending upon variation in the offset.
Such distortion resulting from the amplitude modulation would be removed later by limiting and then FM demodulating the amplitude, as long as the amplitudes and phases of the upper and lower side-bands are transmitted without change in the course of processing the video disc signals.
However, these sidebands are actually subjected to change in the aperture characteristic during the reading operation due to the light beam and to distortion in the phase characteristic of the amplifiers included in the signal processing section, which makes it difficult to transmit signals with complete fidelity. This will involve mixing such distortion as crosstalk into the output of the FM demodulator 9.
As a result, the voice FM signal is mixed partly in the video signal and appears on the reproducing display as a beat interference of about 2 MHz, thus deteriorating the picture quality.
Next, explanation of crosstalk from the video signal to the voice signal will be made.
As has been described above, the size of the transparent or opaque part on the rotary disc will change due to various factors, that is, be larger or smaller than the selected size, during the recording process. Now, assume that the size of the transparent part is larger than the selected size and the size increment corresponds to 30% of the wave length to be recorded on the disc of the black level signal (in this case, the frequency of the FM carrier is 8 MHz) in the video signal. Then, the increment corresponds to 30%.times.9.2/8=34.5% when it is evaluated with reference to the recording wave length of the white level signal (the frequency of the FM carrier at the white level is 9.2 MHz).
Essentially speaking, any change in the duty factors of the transparent and opaque parts recorded on the disc must depend merely on the voice FM signal required to record and reproduce the voice signal, and the duty factor must not depend on the video signal. In fact, the change in the duty factors depends partly on the video signal so that the voice FM signal is also modulated with a base band signal in the video signal, that is, the base band spectrum of about 2 MHz is mixed into the output of the band-pass filter 8 (see FIG. 1). In an extreme case, the level of the base band spectrum is greater than that of the voice signal carrier, which causes noise known as buzz interference during the reproducing operation thereby causing significant deterioration in the quality of the reproduced voice signal.
In addition, there is a mutual relation between the two types of crosstalk produced in the recording and reproducing system. More specifically, there exists an inconsistent problem, that is, a decrease of one cross-talk will cause an increase of the other crosstalk. For example, when the relative recording level of the voice FM signal is increased to decrease the buzz interference in the voice signal, beat interference will increase on the display. On the other hand, if the level of the voice signal is decreased, the beat interference on the display will decrease. The buzz interference in the voice signal will increase correspondingly. For this reason, the types of crosstalk must be counter-balanced, in the actual application. It will be understood from the foregoing that it is difficult in the prior art technique to get a video disc player which meets both the conditions of high picture quality and high voice quality by reducing crosstalk. Further, such a crosstalk problem is regarded as a common problem in a higher frequency system in which a main FM signal and a lower frequency sub-signal are provided in the frequency multiplex mode.